As an introduction to the problems solved by the present invention, consider for example the conventional perimeter alarm system based on laser beam interruption as used to monitor ingress onto a swimming pool apron. Such a system is difficult to initially install and requires considerable maintenance to control the occurrence of false alarms.
Many different physical effects of the installation can independently effect a false alarm. For example, when one infrared laser source is used with several mirrors to create a continuous path around the perimeter to be monitored, the initial alignment of the mirrors is costly. If any one mirror becomes misaligned, through sudden or gradual movement, the beam is interrupted as a false alarm. Correction of misalignment may require use of expensive infrared sensitive equipment. When the several mirrors are aligned sufficiently to remove the false alarm, one or more mirrors may not be positioned to reflect the beam from the center of the mirror. Consequently, the system's tolerance for future misalignment may be lower than expected.
The conventional detector for such a system may raise false alarms in response to light from sources other than from the laser source. Ambient sunlight may impinge upon the detector directly or as reflected by any surrounding surface or mirror. The angle of direct sunlight varies throughout the day and throughout the year to include a very wide range of angles. In addition, sunlight reflects from the surface of water in the swimming pool in an even wider range of angles varying randomly with wind conditions. The amount of background light on which a change is to be detected also varies making false detection more likely. An alignment of mirrors prescribed during installation or maintenance is unlikely to be sufficient for all of the above conditions.
The operator of such a system is exposed to risk of loss unnecessarily. As a result of false alarms, operators of such perimeter monitoring systems may be less likely to respond immediately when an alarm sounds. Failure to timely respond may result in a loss of life or property. When interrupted by a large number of false alarms, the operator may defeat the monitor or the alarm and not reactivate the monitor or the alarm due to operator irresponsibility or forgetfulness.
In view of the problems described above, the need remains in perimeter monitoring systems for higher reliability and lower installation and maintenance costs.